Date: Nov 1, 2012 12:16 AM
Author: Stuart.Palmer@deakin.edu.au
Subject: Testing the significance of difference in set means at different times

I have data from two large sets of samples (100s in one set [a1] and 1000s in the other set [b1]).  a1 has a mean score Ma1.  b1 has a mean score Mb1.  I can evaluate the significance associated with the difference in mean scores [Ma1-Mb1] observed between these two sets.

I also have two similar sets of data collected at a later period - a2 and b2, with means Ma2 and Mb2. Again, I can evaluate the significance associated with the difference in mean scores [Ma2-Mb2] observed between these two sets.

I'm interested in whether there is a way to evaluate the significance of the observed change across the two time periods between the difference observed in the set means? That is, an appropriate method to evaluate the significance of the observed difference in (Ma1-Mb1)-(Ma2-Mb2).

Any sugestions would be greatly appreciated.